2 Anatomy Meninges and Tracts The nervous system consists of two main anterior and posterior horns and joined divisions: the central nervous system by a thin commissure containing the cen- (CNS), consisting of brain and spinal cord, tral canal, which is connected to the and the peripheral nervous system, con- fourth ventricle. The gray matter is sur- sisting of cranial and spinal nerves, and rounded by white matter, which consists their associated ganglia. mainly of ascending and descending Three membranes surround both spi- tracts, and has been divided arbitrarily nal cord and brain: dura mater, arachnoid into anterior, lateral, and posterior mater, and pia mater. The dura mater is a columns. The individual tracts will be tough, fibrous coat that encloses the spinal dealt with in more detail later. column and cauda equina, which is a In the peripheral nervous system, there bundle of nerve roots from the lumbar, 12 pairs of cranial nerves, which leave the sacral and coccygeal spinal nerves. The brain through foramina (apertures) in the dura mater runs rostrally and is continu- skull, and 31 pairs of spinal nerves, which ous beyond the foramen magnum with the leave the spinal cord through vertebral dural meninges, which cover the brain. foramina. There are eight cervical, 12 Caudally, the dura ends on the filum termi- thoracic, five lumbar, five sacral, and one nale at the level of the lower end of the coccygeal pair of spinal nerves. The spinal second sacral vertebra. The dura is sepa- nerves are linked to the cord by dorsal rated from the walls of the vertebral canal (posterior) nerve roots, which carry affer- by the extradural space, which contains ent nerves into the CNS, and ventral the internal vertebral venous plexus. The (anterior) nerve roots, which carry effer- dura extends along the nerve roots and is ent nerves away from the CNS. Afferent continuous with the connective tissue that fibers are also called sensory fibers, and surrounds the spinal nerves. The inner their cell bodies are situated in the swel- surface of the dura is in direct contact with lings or ganglia on the dorsal roots. the arachnoid mater. The arachnoid mater is a relatively fragile, impermeable layer that covers the spinal cord, the brain and spinal nerve roots, and is separated from the pia by the wide subarachnoid space, which is filled with cerebrospinal fluid. The pia mater is a highly vascularized membrane closely ap- posed to the spinal cord. It thickens on each side between the nerve roots to form lateral supports, anchored to the arachnoid, which suspend the spinal cord securely in the center of the dural sheath. The spinal cord is an approximately cylindrical column, continuous with the medulla oblongata, that extends in adults from the foramen magnum to the lower border of the first lumbar vertebra. Struc- turally, the cord contains central gray mat- ter, roughly H-shaped, consisting of the Greenstein, Color Atlas of Neuroscience © 2000 Thieme All rights reserved. Usage subject to terms and conditions of license. Anatomy 3 Greenstein, Color Atlas of Neuroscience © 2000 Thieme All rights reserved. Usage subject to terms and conditions of license..